The Study to Improve Oil Recovery through the Clay State Change during Low Salinity Water Flooding in Sandstones

[Image: see text] Low salinity water flooding is a low-cost enhanced oil recovery (EOR) technology. The mechanism of EOR in a sandstone reservoir is still controversial, and there are many influencing factors. In this study, the effects of salinity (2000, 4000, 8000, and 100,000 ppm), pH (5.5 acidic, 7.0 neutral, and 8.0 alkaline), cation type (Na(+) and Ca(2+)), and clay content (A rock 6.04%, B rock 11.94%) on zeta potential and recovery related to clay swelling were studied. The results showed that the absolute value of zeta potential increased with the decrease of salinity, cation changes from divalent to monovalent, and an increase of the pH value or clay content. The results of the SEM test before and after displacement and the continuous increase of displacement pressure after low salinity water injection show that low salinity water will cause clay swelling and the absolute value of zeta potential increased. The extreme value of recovery appears in the rocks with a high clay content: In neutral and alkaline NaCl solutions, R(I) and PEOR of rock B first increase and then decrease with the decrease of salinity. When the salinity is 4000 ppm, R(I) and PEOR were 8.16 and 34.13% in the neutral state, and 8.50 and 25.00% in the alkaline state, respectively. R(I) and PEOR of other experimental groups increased with the decrease of salinity. The study showed that the displacement pressure increases with the decrease of salinity, which indicates that the proper expansion of clay can improve the recovery of a sandstone reservoir, while the excessive expansion of clay will damage the reservoir and reduce the recovery. Based on the experimental results, the factors and indexes involved in the experiment were analyzed by multiple variance analysis. The result showed that the salinity, cationic type, and pH value have a significant effect on the zeta potential. All factors in the experiment have a significant effect on R(I), salinity, and cationic type, and the clay content have a significant effect on PEOR. The conclusion of this study could guide the design of low-salinity water flooding technology in oil fields.